1. Field of the Invention
The present invention relates generally to an active matrix type image display apparatus. More particularly, the invention relates to an image display apparatus which holds a signal voltage written in a given selected period even out of the selected period and controls electrooptic characteristics of display element by the signal voltage. Further particularly, the invention relates to an image display apparatus performing a multiple gradation display of an image by controlling a holding period of the signal voltage depending upon a video signal.
2. Description of the Related Art
In the recent years, associating with arrival of advanced information society, demand for personal computer, portable information terminal, information communication equipment or composite produce thereof is growing. For these products, thin, light weight and high response display is suitable and a display apparatus, such as self-luminous type organic LED element (OLED) or the like, has been employed.
Pixel of the conventional organic LED display apparatus is constructed as illustrated in FIGS. 21A and 21B. In FIG. 21A, a first thin film transistor (TFT) Tsw 23 is connected at each intersection of a gate line 22 and a data line 21. To the first TFT Tsw 23, a capacitor Cs 25 for storing data and a second thin film transistor Tdr 24 for controlling a current to be supplied to an organic LED 26 are connected.
Waveforms for driving the first TFT Tsw 23 and the second TFT Tdr 24 are as shown in FIG. 21B. A current depending upon a data signal Vsig 28 is applied to a gate electrode of the second TFT via the first TFT which is turned ON by a gate voltage Vgh 29. By a signal voltage applied to a gate of the second TFT, a conductivity of the second TFT is determined. A voltage Vdd applied to a current supply line 27 is divided between TFT and an organic LED element as load element to determine a current flowing through the organic LED element. Here, in a construction where Vsig takes multiple values, it is required that a characteristics of the second TFT is uniform over entire display region of the display apparatus. However, due to non-uniformity of electrical characteristics of the TFT which is formed an active layer by amorphous silicon, difficulty is encountered in satisfying the foregoing demand.
In order to solve this problem, it has been proposed a digital drive system, in which the second TFT is used as switch to take a current flowing through the organic LED element as binary value of ON and OFF. Tone expression is realized by controlling the current to flow the current. One of known example has been disclosed in JP-A-10-214060.
A diagram of driving is shown in FIG. 22. A vertical axis of FIG. 22 represents a position of scanning line in vertical direction and a horizontal axis is a time. FIG. 22 shows driving of the display apparatus for one frame. In driving by the prior art, one frame period is divided into four sub-frames. In each sub-frame, vertical scanning period having a length common to all sub-frames and luminous periods having length weighted with weighting coefficients 1, 2, . . . 24=64, per each sub-frame.
As set forth above, by a system of separating the vertical scanning period and the luminous period, vertical scanning period cannot be used for luminous to shorten luminous period occupied in one frame. In order to certainly obtain luminous period, the vertical scanning period has to be made shorter. However, since ON period of Tsw substantially correspond to vertical scanning period/the number of vertical scanning lines m, sufficiently large vertical scanning period becomes necessary for certainly obtaining ON period in consideration of wiring capacity, resistance and so forth specific to active matrix. For example, in case of displaying of eight sub-frames, about 1 ms of vertical scanning period is expected per one sub-frame. In this case, a period to be used for luminescence becomes about 8 ms which is substantially half of one frame. Furthermore, in such case, it is required that one vertical scan is sixteen times the high speed than normal scanning speed.
This problem may be solved by multiplexing vertical scan to progress vertical scan and luminescence simultaneously. At this time, driving diagram is shown in FIG. 23. FIG. 23 shows an example at 3-bit, in which is shown a condition where three vertical scan and display are progressed. Basic concept of the driving method has been suggested in “Halftone Moving Picture Display by AC type Plasma Display”, Institute of Television Engineers, Display System Seminar Material 11-4, Mar. 12, 1973, and Japanese Patent No. 2954329 applied for active matrix liquid crystal. However, a construction for actually implementing the driving method of vertical multiplexing has not be disclosed.
Upon performing high definition and multiple gradation display using digital data, it becomes necessary to increase operation speed of the driving circuit and to increase circuit scale of the driving circuit according to increase of number of data. Therefore, progress of increasing of display density and increasing of gradation levels to cause increasing of power consumption. As a solution for this, lowering of power consumption is desired.
On the other hand, in the method for controlling On/OFF display per each frame with dividing display period into several sub-frames, lowering of picture quality of the moving picture for admixingly presenting data between series of frames when moving picture display, such as television or the like, is to be performed.